A flat panel display device (FPD) such as an organic EL display (organic electro-luminescence display: OELD), a plasma display panel (PDP) or a liquid crystal display device (LCD) has such a structure that a glass substrate for element, that has a display element formed thereon, and a glass substrate for sealing are disposed to face each other and the display element is sealed in a glass package comprising two such glass substrates bonded. Further, for a solar cell such as a dye-sensitized solar cell, application of a glass package having a solar cell element (dye-sensitized photoelectric conversion element) sealed with two glass substrates has been studied.
As a sealing material to seal two glass substrates together, application of a sealing glass excellent in moisture resistance, etc. is in progress. Since the sealing temperature of the sealing glass is at a level of from 400 to 600° C., properties of an electronic element portion of an organic EL (OEL) element or the like tend to be deteriorated by a heating treatment using a firing furnace. To solve this problem, application of local heating by a laser beam for forming a sealing layer is in progress (refer to Patent Documents 1 and 2). The sealing by a laser beam is carried out, for example, in the following process. First, a sealing material containing a sealing glass and a laser absorbent, etc. is blended with a vehicle to prepare a sealing material paste, the paste was applied on a sealing region of one of glass substrates, and such a coating layer of the sealing material paste is fired to form a sealing material layer. Subsequently, the glass substrate having the sealing material layer is laminated with another glass substrate, and the sealing material layer is irradiated with a laser beam to heat the layer to form a sealing layer.
Use of the sealing by laser irradiation can suppress thermal influence on an electronic element portion, but it has a problem that since it is a process of rapidly heating and rapidly cooling the sealing material layer, bubbles tend to be produced in the sealing material layer. Namely, when the sealing material layer is melted by applying laser heating, temperature-rising and temperature-falling speeds of the sealing material layer are faster than those of heating in a normal firing furnace, and accordingly, bubbles tend to be produced in the sealing layer. Such a bubble in the sealing layer functions as a starting point of peeling or breakage, and causes defective sealing. Bubbles caused by a rapid heating-rapid cooling process of the sealing material layer tend to be produced not only in the case of laser heating but also in a case of a heating step of infrared heating, dielectric heating, induction heating or resistance heating, etc. whereby the temperature-rising speed becomes at least 100° C./min like the laser heating. In particular, since the laser heating causes an extremely high temperature-rising speed, bubbles tend to be produced by the laser heating.
Patent Document 3 describes that in order to suppress generation of bubbles at a time of vacuum sealing, the water content in a sealing glass is limited to be at most 300 ppm. However, there is no consideration as to a sealing step using a rapid heating-rapid cooling process such as laser heating or infrared heating. Further, Patent Document 4 describes that the amount of water in a dielectric paste for plasma display panel is limited to be at most 3 mass % in order to achieve smoothness and uniformity of the coating layer and to improve surface state of a fired layer (dielectric layer). This dielectric layer is fired in a normal firing furnace, and there is no consideration as to a heating step using a rapid heating-rapid cooling process such as laser heating or infrared heating.
Patent Document 5 describes that in a bismuth glass powder produced by water pulverization, water is adsorbed to the glass powder and the adsorbed water is not completely evaporated by laser irradiation and remains to cause bubbles, and that to solve this problem, the bismuth glass powder is produced by solvent pulverization to reduce adsorbed water to thereby suppress generation of bubbles. Namely, the document discloses a technique of reducing adsorbed water by reducing contact of a bismuth glass powder with water.